Vapor-Phase Plotting of Organosilane Chemical Gradients - Langmuir

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Vapor Phase Plotting of Organosilane Chemical Gradients Judith Bautista, Anna Forzano, Joshua M Austin, Maryanne M. Collinson, and Daniel A. Higgins Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01977 • Publication Date (Web): 25 Jul 2018 Downloaded from http://pubs.acs.org on July 28, 2018

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Vapor Phase Plotting of Organosilane Chemical Gradients Judith Bautista†, Anna Forzano ‡, Joshua M. Austin†, Maryanne M. Collinson*, ‡ and Daniel A. Higgins*,† †

‡

Department of Chemistry, Kansas State University, Manhattan, Kansas 66506-0401, United States

Department of Chemistry, Virginia Commonwealth University, Richmond, Virginia 23284-2006, United States ABSTRACT

Vapor phase plotting of organosilane-based self-assembled monolayer (SAM) gradients is demonstrated for the first time.

Patterned SAMs are formed by delivering gas phase

organotrichlorosilane precursors to a reactive silica surface using a heated glass capillary. The capillary is attached via a short flexible tube to a reservoir containing the precursor dissolved in toluene. The proximal end of the capillary is positioned at an experimentally-optimized distance of 30 µm above the substrate during film deposition. The capillary is mounted to a steppermotor-driven X,Y plotter for raster scanning above the surface. organotrichlorosilane

precursors

are

employed

in

this

initial

Two different demonstration:

n-

octyltrichlorosilane and 3-cyanopropyltrichlorosilane. The dependence of SAM deposition on ambient relative humidity, capillary-substrate separation, raster scanning speed, and solvent viscosity and volatility are all explored and optimum deposition conditions are identified. The optimized procedures are used to plot uniformly modified square “pads” and gradients of the silanes. Film formation is verified and the gradient profiles are obtained by sessile drop water contact angle measurements, spectroscopic ellipsometry measurements of film thickness, and by X-ray photoelectron spectroscopy (XPS) mapping. The resolution of the plotting process is currently in the millimeter range and depends on capillary diameter and distance from the substrate surface. Vapor phase plotting affords an unique direct-write method for producing patterned and chemically graded SAMS that may find applications in microfluidic devices, planar chromatography, and in optical and electronic devices.

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Introduction Thin films designed to exhibit chemical and/or physical properties that gradually vary along one or more directions are known as chemical gradients.1-4 These materials find myriad potential applications in diverse fields of research ranging from materials science to biology. They have been used previously in combinatorial materials chemistry,5 high-throughput investigations of molecule-surface interactions,6 catalysis,7 chemical separations,8,9 molecular combing of DNA10 and to drive the motions of liquid droplets,11 vesicles,12 nanoparticles,13 macromolecules,14 and cells.15 Gradient fabrication has been accomplished by a variety of methods, including by vapor diffusion,11,16 solution diffusion,6,15,17 microfluidic mixing,18 ink jet printing,19 photolithography,20 contact printing,21 electrochemical methods,22 dip-coating,23-25 and controlled-rate infusion methods.26,27 Deposition methods that rely upon precursor diffusion alone do not afford any control over the gradient profile or steepness, which is determined by Fick's Laws. Furthermore, few such methods allow for patterning of the film. Unless masking procedures are employed to achieve gradient deposition over small regions, the gradients obtained usually cover the entire substrate surface.

While photolithography20 and contact

printing21 afford control over the gradient profile and can be used to make patterned films, changing the gradient characteristics or film pattern requires the fabrication of a new mask or stamp.

Although both dip-coating23-25 and controlled-rate infusion26,27 methods allow the

gradient profile to be manipulated, neither allows for the deposition of patterned films. Ink jet printing19 is a direct-write method that provides the flexibility required to produce gradients of arbitrary shape, location, and profile, but it requires careful optimization of the ink properties (e.g., surface tension, viscosity and solvent volatility) to achieve proper jetting.28,29 Furthermore, the printing of reactive materials by ink jet methods frequently leads to clogging of the print

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nozzles. Since ink jet printing relies on the deposition of liquid droplets on solid surfaces, dewetting of the substrate and de-mixing of the ink solutions can also be problematic. As an alternative to the aforementioned methods, this report describes the vapor phase plotting of organochlorosilanes for making patterned and chemically graded self-assembled monolayer (SAM gradient) films. To the best of our knowledge, this represents the first report of vapor phase plotting of organosilane materials. Although similar vapor jet printing methods have been employed for the deposition of organic semiconductors,30-32 these methods have not yet been used to fabricate gradients. Vapor jet printing works by passing a heated gas through a hot organic powder or liquid source, often under reduced pressure. The gas carries the molecular species through a nozzle and deposits them onto a cooled substrate. The printing of patterned films with ~ 4 µm spatial resolution has been reported under certain conditions.33 In the present method, organosilane vapor is delivered to a reactive silica surface using a glass capillary positioned just above the surface. The capillary is connected via a short flexible tube to a reservoir containing the precursor liquid. A carrier gas sweeps precursor vapor from the headspace above the reservoir into the tubing and delivers it to the capillary, which provides for localized exposure of the substrate. Unlike vapor jet printing, organosilane vapor phase plotting is performed at room temperature and atmospheric pressure. By raster scanning the capillary above the silica surface, patterned uniform or gradient films that cover a small region of the substrate can be produced. The amount of material deposited can be changed by varying either the raster-scanning speed or the carrier gas flow rate. The ultimate spatial resolution of vapor phase plotting is governed by diffusion and convective dispersion of the gas-phase precursors after they exit the capillary. The size of the smallest features that can be produced is limited by the capillary diameter and its distance above the substrate surface. In this initial

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demonstration, we have achieved millimeter scale spatial resolution using glass capillaries with millimeter inner diameters.

With further development of the method, up to a 100-fold

improvement in plotting resolution should be achievable. Vapor phase plotting is a direct-write method that allows for patterned uniform or gradient films to be prepared over selected substrate regions, without modifying neighboring areas. It is likely to find applications in the fabrication of SAMS for use in planar chromatography, microfluidic devices, electronics, and optical devices. Experimental Considerations Materials SAM films and gradients were prepared from n-octyltrichlorosilane (97%) and 3cyanopropyltrichlorosilane (97%) precursors. The organotrichlorosilanes were obtained from Sigma-Aldrich and were used as received. Each was dissolved in a solvent prior to being transferred to the reservoir. The solvents tested include toluene, n-heptane, and n-butanol. Both microscope coverslips (Fisher Finest Premium, 25 mm x 25 mm) and cut, polished silicon wafers (University Wafer, boron doped, 10 mm x 20 mm) were used as substrates upon which the SAM films and gradients were deposited. Each substrate was cleaned prior to use by exposure to an air plasma for 5 min. As has been demonstrated previously, organosilane films are best formed on substrates already coated with a silica base layer.26 The base layer provides a uniformly reactive surface with sufficient silanol sites to allow for efficient attachment of the organotrichlorosilanes.34 Base layers were deposited from a sol comprised of tetramethoxysilane (TMOS), ethanol (200 proof), and 0.1 M HCl in volume fractions of 0.94:94.0:5.06 (TMOS:ethanol:0.1M HCl). In preparation of this sol, the components were first added to a clean glass vial. The sol was then stirred for 1 h and aged for another 23 h in a desiccator prior

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to use. Each base layer was formed by spin coating (Specialty Coating Systems, Inc., P-6000) a 150 µL volume of the sol onto the substrate at 5000 rpm for 30 s. Base layers prepared in this manner had thicknesses of 16 ± 5 nm on average, as determined by spectroscopic ellipsometry. The base-layer-coated substrates were stored in a desiccator for at least 24 h prior to use and were subsequently exposed to an air plasma for 2 min prior to vapor plotting. Methods Film deposition and gradient formation were verified by sessile drop water contact angle (WCA) measurements, spectroscopic ellipsometry, and X-ray photoelectron spectroscopy (XPS) mapping. The same methods also allowed for the gradient profile to be recorded. For WCA measurements, 0.1-0.5 µL droplets of pure water (18 MΩ·cm, B-pure, Barnstead) were placed on the SAM films at several locations, using a microliter syringe (Hamilton).

A home-built

instrument equipped with a CCD camera (ProVIDEO, CVC-140) and zoom lens (Navitar) was used to photograph the individual water droplets. A plugin for the freely available ImageJ software35 was used to determine the contact angle from each photograph. Silica base layer and SAM film thicknesses were measured using an α-SE spectroscopic ellipsometer (J.A. Woollam Co., Inc.). All film thickness data were acquired from samples deposited on silicon wafers. The use of silicon substrates ensured a strong optical reflection from the film-substrate interface, affording the measurement precision needed to determine the thickness of sub-monolayer films.

To avoid errors in the measured thickness caused by

condensed water layers, all ellipsometric measurements were made under a dry nitrogen atmosphere inside a Plexiglas chamber. The chamber was purged with nitrogen for at least 30 min prior to each set of measurements, and the purge was maintained throughout each experiment. In fitting the ellipsometry data, each film was modeled as a transparent film on

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silicon, having refractive index n = 1.457 (at 633 nm). The optical dispersion of both the silica base layer and organosilane films were modeled by the Cauchy Equation. In all cases (both base layer and organosilane-coated base layer), the films were treated as a single layer on the substrate, because the refractive indexes of the films are all very similar.36 To determine the organosilane layer thickness, the base-layer thickness was first measured prior to vapor plotting of the organotrichlorosilane. The base-layer thickness was measured at a series of positions separated by 2 mm spacings along the substrate. The SAM film was then deposited atop the base layer and the film thickness was remeasured at each of the previous locations (to within ± 1 mm). The SAM film thickness was obtained as the difference between the full film thickness and that of the base layer. XPS mapping experiments were employed to verify the chemical composition of the SAM films and gradients.

XPS data were acquired using a Thermo Fisher ESCAlab 250

imaging spectrometer. This instrument uses an Al Kα source (1468.68 eV). XPS spectra were acquired at 1-2 mm spacings on each film, using a 500 µm spot size, 50 eV pass energy, and 0.100 eV step size. Analysis of film properties was restricted to the N(1s) region of the spectrum for the cyanopropyltrichlorosilane-derived SAM films. The presence of adventitious carbon made XPS characterization in the C(1s) region difficult. However, the C(1s) data were also collected to allow for the N(1s) data to be corrected for charge shifts. In this case, the data were all shifted so that the C(1s) peak for adventitious carbon appeared at 284.6 eV. The area under the nitrile N(1s) peak was determined by subtracting the background, fitting each spectrum with two Gaussian functions,37 and determining the area under the appropriate peak from its amplitude and width. Results and Discussion Vapor Phase Plotting 6


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SAM films and gradients were prepared using a simple, inexpensive vapor phase plotter that was designed and constructed in-house. The critical working components of the vapor plotter include a precursor reservoir, a glass capillary for delivery of the precursor vapor to the substrate surface, and an X,Y plotter. Figure 1 shows a schematic diagram and a photograph of the apparatus. Briefly, vapor plotting is accomplished by first loading the precursor reservoir with organosilane solution. An inert carrier gas is then used to sweep organosilane vapor from the reservoir into the capillary, which is connected to the reservoir via flexible tubing. The capillary is mounted to the X,Y-plotter, allowing for it to be raster scanned above the substrate surface. The working end of the capillary is positioned a few tens of micrometers above the substrate, allowing for local regions to be exposed to silane vapor in a controlled manner. The entire apparatus is housed within a sealed deposition chamber that allows for the ambient atmosphere to be controlled during plotting. The entire vapor phase plotting process is described in more detail, below. As noted above, delivery of the precursor silanes to the substrate surface requires the use of a carrier gas. Dry nitrogen was employed in these studies. The carrier gas was first passed through the precursor reservoir and then into the flexible tubing connecting the reservoir and capillary. The carrier gas flow rate was maintained at 3.3 ± 0.1 ml/min in these experiments. The volume of the headspace in the precursor reservoir was ~ 5 ml. Two types of flexible tubing were tested in this initial demonstration. They include Silastic tubing (Dow Corning, 1.57 mm ID, 3.18 mm OD) and Teflon tubing (Optimize Technologies, Inc., 1.8 mm ID, 3.2 mm OD). The Teflon tubing was concluded to be best suited for this application because it is more compatible with the solvents and silanes employed.38

Some evidence for accumulation of

residual silanes within the tubing and between runs was observed when the Silastic tubing was

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used, particularly at high precursor concentrations. Because Teflon tubing is less elastic, a short segment (~ 1 cm) of the Silastic tubing was used to join the Teflon tubing to the glass capillary.

Figure 1. Deposition chamber for vapor phase plotting of SAM films and gradients. The inset shows a photograph of the capillary, substrate, reservoir and sample stage. The capillaries selected for use were melting point capillaries (Kimble Chase, 1.5 mm ID, 1.8 mm OD, 4.5 cm L). Each capillary was wrapped in Nichrome wire (Omega, 0.16 mm diam., 56 Ohm/m) to allow for heating to ~ 35° C, as measured with an infrared thermometer. Heating of the capillary was required to prevent condensation of the precursor at its outlet when high precursor concentrations were employed. The glass capillary was mounted on the stepper motor driven X,Y plotter with its long axis oriented perpendicular to the substrate surface, as shown in Figure 1. The plotter stage employed was obtained from Wave Dynamics, and had a 320 mm x 240 mm total travel, with a maximum working speed of 450 mm/s and 300 mm/s in the two directions.

The raster scanning speed employed during deposition ranged from 0.4 - 175

mm/min in the fast-scan direction.

The substrate was positioned beneath the capillary by

mounting it on a tilt stage (ThorLabs, KM100B) attached to a linear translation stage (Newport, 460A). This mounting procedure allowed for the substrate to be reproducibly positioned beneath the capillary with micrometer precision. The X,Y plotter was controlled by software written in-

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house in the Labview (National Instruments) programming environment. The entire apparatus was housed inside a transparent plastic box (the deposition chamber) to better control the ambient humidity. The humidity within the box was raised when necessary either by bubbling air recirculated from within the box through a water-filled flask, or by use of a miniature ultrasonic humidifier. Strong convective air currents were required to achieve optimum spatial resolution during vapor plotting. These air currents served to rapidly remove and dilute residual unreacted precursor vapors from near the capillary/substrate junction. A blower attached to 10 cm diameter tubing was used for this purpose. The blower continuously recirculated the air back into the deposition chamber at a distance of ~ 0.35 m from the substrate. A mechanical damper was used to control the flowrate of air through the blower. A marked degradation in plotting resolution was observed if the blower was not turned on. The deposition chamber was vented into the fume hood system prior to removal of the sample, following each deposition. Optimization of Plotting Conditions The extent of surface modification was found to depend upon a number of different parameters, including the rate at which the liquid precursor evaporates into the vapor phase inside the precursor reservoir. The rate of precursor evaporation is dependent upon the boiling point of the precursor and its heat of vaporization, but it also depends on its rate of diffusion to the liquid-vapor interface (the precursor solutions were not stirred). According to the StokesEinstein relation, the rate of diffusion scales inversely with solution viscosity.

Therefore,

solvents of low viscosity were employed for diluting the precursor silane. To identify a suitable solvent, n-octyltrichlorosilane was alternately dissolved in toluene (η = 0.560 cP), n-dodecane (η = 1.383 cP) and n-butanol (η = 2.544 cP).39 Square SAM "pads" 3 mm x 3 mm in size were then plotted under otherwise identical conditions. Films plotted from toluene solutions gave the

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highest WCAs (~ 71o) while those plotted from n-dodecane gave somewhat smaller values (~ 64o) and n-butanol gave the lowest WCAs (~ 41o). Based on these results, toluene was selected for use as the diluting solvent in the remainder of the work presented below. The low WCAs obtained when n-butanol was employed could result from reaction of the chlorosilane with the alcohol, in addition to slow precursor diffusion in solution. The dependence on precursor evaporation rate could lead to variations in film properties with changes in the reservoir design and carrier gas flow rate. A fixed reservoir design and constant gas flow rate (see above) were employed to help avoid such difficulties. As has been previously reported,40 the deposition of organotrichlorosilanes from the vapor phase was found to be strongly dependent upon the relative humidity of the environment within the deposition chamber. It is believed that the moisture dependence of film growth results from the participation of surface-adsorbed water in cross-linking of the trichlorosilane precursors.41,42 In order to identify the optimum level of moisture required for efficient film deposition, 3 mm x 3 mm square n-octyltrichlorosilane pads were plotted at a series of different relative humidities, with all other factors held constant.

Figure 2A,B plot the SAM film

thicknesses and WCAs measured from these pads as a function of the relative humidity (RH). These data show that the highest WCAs were obtained at humidities above ~ 30% RH. The film thickness data also provide clear evidence of humidity dependent variations in the SAM film surface coverage. The mean film thickness fell from ~ 1 nm at humidities above 50% RH to ~ 0.7 nm at a humidity of ~ 23% RH. A full monolayer of well-packed octylsilane molecules is expected to be ~ 1.1 nm thick.40 Based on these results, all films described in the remainder of this report were prepared at humidities of ~ 45 ± 5% RH.

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Figure 2. A) Film thickness for n-octyltrichlorosilane monolayers prepared under different relative humidities. B) Water contact angles measured for the same films. The concentration of silane in the deposition reservoir was 10 vol % in toluene solution. The capillary-substrate separation was 30 µm. The step time during raster scanning was fixed at 3.6 s. C) Water contact angle measured as a function of n-octyltrichlorosilane concentration (vol %) in toluene. The SAM pads characterized were deposited at ~50% RH with a capillary-substrate separation of 30 µm and a stage speed ~ 0.44 mm/min. D) Water contact angle as a function of stepper motor step time (top axis) and 1/stage speed (bottom axis). Both the step delay and stage speed correspond to motion along the fast raster scanning axis. The solid lines have been added to better depict trends in the data. The fast rise in WCA at short step times is consistent with a fast kinetic process for the initial reaction with the surface. The error bars depict the 95% confidence interval on each value for n = 12, 5, 5 and 5 in panels A) - D), respectively. The distance between the capillary and substrate was found to impact both the plotting 11


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resolution and the ultimate SAM surface coverage under otherwise identical conditions. Here, the capillary-substrate distance was adjusted using the linear stage upon which the sample was supported (see Figure 1), and was measured from magnified images of the junction obtained using a CCD camera. In these experiments, several square SAM pads (3 mm x 3 mm in size) were again plotted and the WCAs measured for each. These data are given in the Supporting Information (Figure S1). The results show that the WCA was greatest (~ 71o) at a capillarysubstrate separation of ~ 30 µm. The WCA decreased slightly (by ~ 5 – 10o) as the capillary was moved either closer to, or further away (out to ~ 90 µm) from the substrate. The decrease in WCA at shorter capillary-substrate separations is attributed to the increased velocity of the vapor exiting the capillary as it is brought closer to the surface. This effectively reduces the substrate exposure time and decreases the degree of modification. At greater distances, the effective concentration of precursor in the vapor phase is reduced by removal and dilution of the precursor by the convective air currents created by the blower, again leading to a reduction in surface coverage. Based on these results, a separation of 30 µm was selected for use in all experiments. As noted above, the precursor silanes were diluted in a suitable solvent prior to deposition. Without dilution of the liquid precursor, its delivery to the capillary and substrate surface was frequently too rapid, leading to visible condensation of the precursor in the capillary and on the substrate surface. Therefore, the concentration of organotrichlorosilane in the toluene solution is also a critical parameter that must be optimized to obtain good films in a reasonable amount of time. Figure 2C plots the WCA values measured for a series of 3 mm x 3 mm square SAM pads printed using a range of n-octyltrichlorosilane concentrations. The concentration is given in volume percent, with toluene as the solvent. The WCA values were found to increase with increasing precursor concentration from ~ 1 vol % (WCA ~ 50o) to 100 vol % (WCA ~

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92o). Control experiments performed with pure toluene in the reservoir produced surfaces with WCAs of < 10o. Finally, the reaction of the organotrichlorosilane with the substrate surface occurs at a finite rate,34 and hence, the length of time the substrate is exposed to the vapor also plays an important role in determining the surface coverage and the WCA achieved. The influence of reaction kinetics on the surface coverage was explored by varying the rate at which the capillary was raster scanned across the surface during silane deposition. Square SAM pads 3 mm x 3 mm in size were again printed in these studies. The pads were printed as a square array on individual substrates to produce patterned surfaces. As stepper motors were used to move the plotting stage (see Figure 1), the rate of capillary motion was adjusted by simply changing the time between motor steps. The time between steps in the fast-scan direction was varied between 9 ms and 3.6 s. The WCAs obtained for SAM pads deposited from 10 vol % octyltrichlorosilane are plotted as a function of the step time in Figure 2D (top axis) and as a function of reciprocal stage speed in min/mm (bottom axis). Here, the stage speed corresponds to the rastering speed along the fast-scan direction. Replicate measurements are included in Figure 2D to demonstrate the reproducibility of the WCA values obtained. These data reveal a biexponential rise in the WCA as the time between steps increased. This observation is consistent with an initial fast reaction of the silanes with the substrate surface, followed by a much (~ 50-fold) slower deposition occurring at longer times.

Indeed, early studies of chlorosilane reaction kinetics on silica

surfaces show similar behavior, with the kinetics exhibiting a ~ 1.5 order dependence on the density of reactive surface sites.34 The complex deposition kinetics are also consistent with what has been observed for other silanes deposited in a time-dependent manner from solution.26,27

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It is noteworthy that the WCA values shown in Figure 2 never reach the ~ 105o value reported previously for octylsilane monolayers deposited by vapor diffusion methods.40 Rather, films that exceed a monolayer in thickness (> 1.1 nm) were found to yield WCAs of ~ 90o (see Figure 2C). The exact origins of the lower WCAs observed for films prepared by vapor phase plotting are unknown at present but are believed to reflect molecular disorder in the films caused by the rapid delivery and removal (i.e., short residence times) of the silanes on the substrate. The kinetics of silane removal are likely similar to the surface reaction rate and this would result in the initial attachment of silanes at relatively low surface coverages. In this case, the hydrocarbon chains are likely to be oriented randomly, leading to films that are more disordered than ideal SAMs.43 Future studies will address this issue further. The

rate

of

precursor

deposition

was

also

investigated

for

the

3-

cyanopropyltrichlorosilane precursor. For this purpose, square pads 3 mm x 3 mm in size were again plotted using a series of different raster scanning rates. Figure S2 shows the WCAs obtained. In this case, they approached ~ 57o, as has been observed previously for vapor deposited cyanopropylsilane films.40 The maximum film thickness was somewhat smaller than for the octylsilane, yielding a value of 0.6 nm (data not shown). The smaller film thickness is expected from the molecular structure of the precursor and is close to that predicted for a full monolayer. It should be noted that both the octylsilane and cyanopropylsilane films were characterized as deposited and were not rinsed prior to WCA, thickness, or XPS measurements. However, in each case, characterization of the films was delayed for at least 3 h after film preparation. Film stability was verified by several methods. In one set of experiments, the plotted films were sonicated in 18 Mohm·cm water for 3 min, at a temperature of 65 oC. They

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were subsequently rinsed with water and blown dry in a stream of nitrogen. In another case, the films were sonicated in isopropyl alcohol for 3 min, then in acetone for 3 min, then rinsed with acetone and blown dry with nitrogen. Finally, some films were soaked in room temperature water for a minimum of 16 h. In all cases, the plotted silane films yielded similar or slightly higher (increased by ~ 6o) WCAs after these treatments, while neighboring (unmodified) regions yielded WCAs of < 10o. Plotting of Organosilane Gradients Organosilane gradients were plotted using the conditions determined in the above experiments. The previous data were used to select the optimum humidity, silane concentration, and capillary-substrate separation. For both precursors, humidities of 45 ± 5% RH were used. Organotrichlorosilane precursor concentrations of 10 vol % were employed except when otherwise noted. The capillary was maintained at a distance of 30 µm above the substrate in all cases. Gradients were obtained by gradually decreasing the step time from several seconds to a few milliseconds. The step time was held constant as each line was plotted along the fast-scan (gradient width) direction and was gradually decreased along the slow-scan (gradient length) direction. The variations in step time along each gradient were determined separately for the octylsilane and cyanopropylsilane precursors, as described in Supporting Information. Gradients plotted using the n-octyltrichlorosilane were 7.5 mm in length and 3 mm in width, while those prepared using 3-cyanopropyltrichlorosilane were 9 mm in length and 5 mm in width.

Larger

area gradients were plotted in the case of the cyanopropylsilane to facilitate XPS data collection. Figure 3 plots film thickness and the associated WCAs for the n-octyltrichlorosilane and 3-cyanopropyltrichlorosilane gradients. Two replicate gradients were prepared and characterized to verify the reproducibility of vapor plotting for each precursor (see blue and red data points in

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Figure 3). Note that values are only shown for the gradient portion of each coated substrate. As

Figure 3. Ellipsometric film thickness and sessile drop WCA measured along the same gradient films plotted using A), B) n-octyltrichlorosilane and C), D) 3cyanopropyltrichlorosilane precursors. The optimum plotting conditions identified above were used in preparing these films. The blue and red data points depict results from two replicate gradients in each case. All gradients shown here were plotted from 10 vol % silane in toluene. The solid lines have been added to better depict trends in the data. The error bars depict the 95% confidence intervals for n = 3 measurements in panels A) - D). expected, the film thickness and WCA values are greatest at the highly modified end of each gradient, with their values decreasing monotonically as a function of position along the gradient length. Water droplets placed either at the very top of the gradient (i.e., the highly modified end) or along the two sides (i.e., the edges) of each gradient spontaneously moved away from the

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gradient towards unmodified substrate regions. In fact, the highly modified ends and sides (i.e., edges) of each gradient comprise gradients of their own, having very steep profiles over 1-2 mm distances (see below for a discussion of plotting resolution). XPS mapping experiments were performed to confirm that the surfaces actually comprised gradients in the expected functional groups. XPS studies were performed only for the gradients derived from 3-cyanopropyltrichlorosilane, for which the N(1s) peak produced by the nitrile nitrogen could be detected. It was concluded that too much adventitious carbon was present to allow for mapping of the gradients prepared from n-octyltrichlorosilane. Figure 4 plots the XPS results obtained from a gradient prepared using 100% 3cyanopropyltrichlorsilane. Figure 4A shows the associated N(1s) XPS spectra as a function of position along the gradient. These data reveal the presence of two N(1s) peaks. The largest peak, centered at ~ 400 eV, decreased with position along the gradient, while the smaller peak, centered at ~ 402 eV, showed little variation with position. The former is attributed to the nitrile nitrogen37,44 while the latter is assigned to adventitious nitrogen contamination. In order to better assign the peak positions and to quantify the amount of nitrile nitrogen present, all spectra were fit to two Gaussian functions.37 Figure 4B depicts the two components obtained by fitting the data acquired near the low nitrile end of the gradient. This analysis yielded a nitrile nitrogen binding energy of 400.2 ± 0.5 eV, with a binding energy of 402.3 ± 0.7 eV for the adventitious nitrogen. Figure 4C plots the area (in arbitrary units) under the nitrile N(1s) peak (400.2 eV). This gradient was plotted so that its highly modified end was positioned at 5 mm, with the gradient extending to the 14 mm position. The gradient profile depicted in Figure 4C is different from that obtained by ellipsometry in Figure 3C.

While the causes of this difference are presently

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unknown, possible reasons are discussed further, below. It is noteworthy that the precursor

Figure 4. N(1s) XPS data for a vapor plotted 3-cyanopropyltrichlorosilane gradient. The reservoir was filled with 100 vol % cyanopropylsilane during plotting. A) N(1s) spectra recorded along the gradient, starting from the highly modified end (top, 5 mm position). Each spectrum was fit to a two-component Gaussian. Each is shifted by 1500 counts/s from its neighbors to aid in viewing. B) N(1s) spectrum at the low cyanopropyl end of the gradient and its two-component fit (blue and black lines). C) Nitrile nitrogen (400.2 eV) peak area as a function of position along the gradient, which begins at ~ 5 mm and ends at ~ 14 mm. Error bars depict the error in peak area obtained by fitting each spectrum. The solid line and gradient model have been added to better depict the trend in the data. reservoir was filled with 100 vol % cyanopropyltrichlorsilane during preparation of the film

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shown in Figure 4C while 10 vol % silane was used for the one in Figure 3C. A higher concentration of silane was required to produce a film that could be adequately characterized by XPS. An initial quantitative estimate of the spatial resolution achievable by vapor phase plotting may be obtained from the data shown in Figure 4. Positions between 0 and 4 mm correspond to the nominally unmodified silica base-layer-coated substrate. As expected, an abrupt increase in nitrile peak area was observed at ~ 5 mm, with a gradual decrease in nitrile coverage occurring from that point out to ~ 17 mm. The transition from unmodified base layer to fully modified surface at the high nitrile end of the gradient spanned ~ 4.8 mm (see Figure 4D). The spatial resolution was taken to be one half this distance, or ~ 2.4 mm. The finite resolution of vapor plotting also leads to the observed extension of the gradient beyond the predicted 14 mm position. The plotting resolution is expected to depend on capillary diameter and its distance from the substrate surface. Preliminary results (data not shown) support this conclusion. The millimeter resolution observed here represents only an initial demonstration of what might be achieved by vapor phase plotting. Modifications to the method expected to improve the resolution are already under development.

These modifications include

implementation of capillaries having smaller inner diameters, and the development of methods to more quickly remove residual precursor from the capillary-substrate junction. It is expected that as much as a 100-fold improvement in resolution could be achieved in the future. A primary advantage of vapor phase plotting is its ability to prepare gradients of arbitrary profile. For example, gradients that yield a linear decrease in WCA with position could be produced, as could gradients that instead exhibit a linear decrease in film thickness, as reflected by either the ellipsometry data or XPS peak area. Achieving the desired profile in a particular

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parameter requires careful calibration of the results obtained under a range of different plotting conditions (e.g., step time). An appropriate expression relating e.g., WCA to step time can then be developed and implemented during the plotting process to achieve the desired gradient profile. Indeed, the data shown in Figure 3 were obtained from experiments in which this process was tested. The exact functional form of the step time profiles employed along the slowscan direction in preparation of the octylsilane and cyanopropylsilane gradients are shown in Supporting Information (Figures S3,S4). A description of how they were obtained from the data shown in Figure 2D and Figure S2 is also given. The curved (rather than linear) appearance of the WCA plots shown in Figure 3 reflect the complexities of the calibration process, the origins of which continue to be explored. Similar problems lead to the curved appearance of the XPS data in Figure 4C. Obtaining a linear gradient here would require prior calibration of the XPS peak area with respect to step time. The different profiles observed in the ellipsometry and XPS data (compare Figures 3C and 4C) for the cyanopropyl gradients may reflect the need for a different calibration at different precursor concentrations. However, it is more likely the linear decrease in film thickness observed by ellipsometry is simply an artifact of enhanced averaging and reduced spatial resolution, as the ellipsometer employs a 2 mm x 5 mm elliptical spot of light. Conclusions The vapor phase plotting of reactive organochlorosilanes was demonstrated for the first time. This method allows for patterned self-assembled monolayers and chemical gradients to be prepared from organosilane precursors in a direct-write manner. The films thus prepared were characterized by spectroscopic ellipsometry, sessile drop water contact angle measurements, and by XPS mapping. The results demonstrated that monolayer-to-submonolayer films and gradients

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could readily be obtained. This study represents only an initial demonstration of the method. Work to improve the plotting resolution and to afford better control over gradient profiles is presently underway. A broader range of precursors will also be tested in the future. Vapor phase plotting affords a new route to producing patterned and chemically graded self-assembled monolayers of arbitrary size and shape over pre-defined regions of larger substrates. The films obtained have myriad potential applications in controlling the motions and assembly of living cells, liquid droplets, vesicles, and nanoparticles. Vapor phase plotting may also be useful in the production of patterned coatings for lab-on-a-chip, optical, or microelectronic devices. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.XXXXXXX. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected]

ORCID Maryanne M. Collinson: 0000-0001-6839-5334 Daniel A. Higgins: 0000-0002-8011-2648

Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS

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The U.S. National Science Foundation (DMR-1404805 and DMR-1404898) provided support for this work. Takashi Ito is thanked for providing access to the spectroscopic ellipsometer. Ron Jackson and Tobe Eggers are thanked for their assistance in constructing the vapor plotter. REFERENCES (1) (2) (3) (4) (5)

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